Shinarump Formation

[kgbudge]

Here's an unusual bit of petrified wood.

 

This is from the Nacimiento Mine near Cuba, New Mexico. The mine has been closed for some time but occasionally collectors are allowed in. (The land has reverted to the Forest Service, which is cleaning up the aftermath of an abortive attempt to extract copper using sulfuric acid leaching.) The mine is a huge point bar deposit of wood in the Shinarump Formation (Triassic) that was permineralized with copper minerals. The black here is mostly chalcocite, copper sulfide, which in a few spots has been oxidized to blue or green azurite or hematite. At the mine itself, you see black fragments of the chalcocite-mineralized wood with lots of blue-green staining around it, as well as occasional wood fragments that were silicified rather than copper mineralized.

 

 

 

At left is a chunk that was permineralized with copper; at right is a twig that is more conventional silica permineralization. The difference seems to be in the original state of the wood. It appears much of the wood was partially burned, and the charcoal was more likely to permineralize with copper minerals than silica.

 

Cool as copper permineralization is, the fact that it's charcoal rather than pristine wood means this is of more mineralogical interest than paleontological. Still, though y'all might enjoy.

 

[Oxytropidoceras]

There is an interesting field trip report about the Nacimiento Mine in "Wanderlusting the Nacimiento Mine," It was posted on October 29, 2016.

 

Also there is:

 

Talbott, L.W., 1974. Nacimiento pit, a Triassic strata-bound copper deposit. In New Mexico Geological Society Guidebook, 25th Annual Field Conference (pp. 301-303).

 

Another PDF file of Talbott (1974)

 

Finally, there is:

 

Woodward, L.A., Kaufman, W.H., Schumacher, O.L. and Talbott, L.W., 1974. Strata-bound copper deposits in Triassic sandstone of Sierra Nacimiento, New Mexico. Economic Geology, 69(1), pp.108-120.

 

Yours,

 

Paul H.

Edited January 17, 2022 by Oxytropidoceras
added reference

[DPS Ammonite]

How do you know that the wood was charcoal and not coalified? Can you tell by looking at it? It seems to me that small pieces might be charcoal. It would be hard for large pieces of charcoal limbs and logs to be preserved without crumbling. 

 

What does everyone think? How do you tell fossilized charcoal and coal apart? What occurs at Cuba, New Mexico?

[kgbudge]

Some coal (inertinite) is thought to be charcoal.